Remote Sensing of Environment 169 (2015) 205–211

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Remote Sensing of Environment

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Detection of pollution outflow from Mexico City using CALIPSOlidar measurements

J. Kar a,b,⁎, M.A. Vaughan b, Z. Liu a,b, A.H. Omar b, C.R. Trepte b, J. Tackett a,b, T.D. Fairlie b, R. Kowch a,b

a Science Systems and Applications Inc., Hampton, VA, USAb NASA Langley Research Center, Hampton, VA, USA

⁎ Corresponding author at: Science Systems and AppliHampton, VA 23681, USA.

E-mail address: jayanta.kar@nasa.gov (J. Kar).

http://dx.doi.org/10.1016/j.rse.2015.08.0090034-4257/© 2015 Elsevier Inc. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 April 2015Received in revised form 3 August 2015Accepted 12 August 2015Available online xxxx

Keywords:CALIPSO lidar measurementsMexico City aerosolRegional scale pollution outflow

Wepresent the evidence of regional scale outflow of particulate pollution fromMexico City usingmeasurementsfrom the space borne CALIPSO lidar. The vertically resolved results are presented for winter months when thelarge scale biomass burning from nearby areas is minimized, and the aerosol loading is dominated by anthropo-genic outflow from the city. The particulate depolarization ratio in the outflowing plume has high values and re-flects the influence of mixing of the urban pollution with the ubiquitous dust around the city. This is consistentwith the results from previous field campaigns in the city and leads to polluted dust being the dominant aerosolsubtype as identified by the CALIPSO algorithm. A first order estimate of themass flux on two episodes using theaerosol extinction profiles from CALIPSO indicates outflow of several hundred tons per day.

© 2015 Elsevier Inc. All rights reserved.

1. Introduction

It is estimated that globally about 54% of world population lived inurban conglomerations in 2014 which is projected to rise to 66% by2050 (UN, 2014). The large number of megacities in the developingworld attests to this. Air quality in these megacities has come underclose scrutiny in recent years. In particular, surface ozone and aerosolconcentration levels have been observed to be very high in some ofthese megacities which are both detrimental to human health. In fact,megacities can influence regional air quality by exporting various speciesof gases and particulate matter and a pollution potential index has beendeveloped to gauge this influence on the basis of three-dimensionaltransport model simulations (Lawrence, Butler, Steinkamp, Gurjar, &Lelieveld, 2007). There are important differences between export of gas-eous species and aerosols as the latter undergo deposition processes andare preferentially transported horizontally because of scavenging pro-cesses occurring during convective lofting (Kunkel, Tost, & Lawrence,2013). Lagrangian simulations also indicate latitude to be the most im-portant factor for the regional scale transport effects (Cassiani, Stohl, &Eckhardt, 2013).

One of the largest megacities in the world is the Mexico City Metro-politan Area (MCMA, 19.43°N, 99.13°W), which has some 20million in-habitants and is the dominant source of anthropogenic pollution in theregion (Molina, Kolb, de Foy, Lamb, et al., 2007). The unique topographyof the area, with mountains on three sides of the city (~2200 m above

cations Inc. (SSAI), NASA LaRC,

mean sea level), traps the pollution in the city during the early morninghours and vents it in the afternoon through thermally driven processes(de Foy, Burton, Ferrare, Hostetler, et al., 2006, 2011; de Foy, Varela,Molina, & Molina, 2006; de Foy, Krotkov, Bei, Herndon, et al., 2009;Molina et al., 2007; Molina, Madronich, Gaffney, Apel, et al., 2010). Ad-ditionally, MCMA is subject to emissions from the Popocatepatl volcano,located south east of the city (de Foy et al., 2009; Raga, Kok,Baumgardner, Baez, & Rosasa, 1999). As such the emissions and evolu-tion of various constituents affecting the air quality in MCMA havebeen the focus of several fieldmissions in the past two decades with co-ordinated airborne and ground measurements (Molina et al., 2007,2010). In particular, the evolution of the gaseous and aerosol speciesdownwind from MCMA was studied extensively by the MIRAGE-Mexand MILAGRO/INTEX-B field missions (Molina et al., 2010). It has beenrealized that MCMA potentially affects the air quality and troposphericchemistry in the region.

From the various field missions and extensive ground based obser-vations, the aerosol environment in MCMA has been determined to bea complex mixture of dust, inorganic and organic matter with some50% being secondary organic aerosols (de Foy et al., 2011). Whilethese special campaigns have contributed significantly to our under-standing of aerosol transport processes in MCMA, they are necessarilylimited in their scope, both spatially and temporally. A fuller and com-plementary perspective can be obtained from space borne instruments.However the aerosols from MCMA and their evolution or export overregional scales have not been studied extensively using satellite dataas of now. As part of the MILAGRO/INTEX-B campaign, aerosol opticaldepth (AOD) retrieved from MODIS and OMI were used to comparewith the airborne measurements of the urban plume (Livingston,Redemann, Russell, Torres, et al., 2009; Redemann, Zhang, Livingston,

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Russell, et al., 2009). However no vertical information about the aerosolplume could be obtained from these total column passive sensor re-trievals. Space borne lidars can provide highly accurate vertically re-solved measurements of aerosol loading in both the troposphere andstratosphere. The Cloud Aerosol Lidar Infrared Pathfinder Satellite Ob-servations (CALIPSO)mission has been providing high quality verticallyresolved measurements of atmospheric particulates globally since June2006 (Winker, Pelon, Coakley, Ackerman, et al., 2010). In this paper weuse the CALIPSO measurements near MCMA to characterize theregional-scale pollution outflow from the city. The MCMA region isalso subject to emissions from intense biomass burning that takesplace in spring every year in the Yucatan peninsula (Yokelson,Urbanski, Atlas, Toohey, et al., 2007). In order to delineate primarilythe anthropogenic influence, we restrict our study to the dry wintermonths, from November to February, when the biomass burning influ-ences are at their minimum. During winter the cloud occurrences arealso expected to be lower than in summer, providing relatively moreclear sky aerosol observations. Further, anthropogenic pollution levelsalso reach a maximum during winter with more frequent thermal in-versions (Molina et al., 2007). Modeling efforts also indicate the poten-tial for more downwind export in winter (Kunkel et al., 2013). In thisshort note, we use the CALIPSO lidarmeasurements to derive the opticalproperties in the winter outflow and also provide a first order estimateof the amount of outflow.

2. Data

We use the global aerosol products retrieved frommeasurements ofthe Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) instru-ment aboard the CALIPSO satellite since 2006 (Winker, Hunt, & McGill,2007; Winker et al., 2010). The primary level 1 measurements are theattenuated backscatter profiles, polarized parallel and perpendicularto the transmitted laser pulses at 532 nm and the total attenuated back-scatter profiles (i.e., parallel and perpendicular components combined)at 1064 nm. In this studywe use the level 2 5-km resolution layer prod-ucts and profile products from versions 3.01 (June 13, 2006 to October31, 2011) and 3.02 (November 1, 2011 to February 28, 2013) of theCALIPSO data (Vaughan, Pitts, Trepte,Winker, et al., 2014).With respectto earlier releases, this version provides significant improvements in theareas of aerosol layer detection, boundary layer cloud clearing andcloud-aerosol discrimination. The CALIOP level 2 data, which are de-rived from the level 1 data using a number of complex algorithms(Liu, Vaughan, Winker, Kittaka, et al., 2009; Omar, Winker, Kittaka,Vaughan, et al., 2009; Vaughan, Powell, Kuehn, Young, et al., 2009;Winker, Vaughan, Omar, Hu, et al., 2009; Young & Vaughan, 2009),have been assessed in a number of papers. McGill, Vaughan, Trepte,Hart, et al. (2007) evaluated the minimum detectable backscatter coef-ficient for the CALIPSO retrieval scheme, and found it to be “in excellentagreement with theoretical values predicted prior to launch”. Rogers,Vaughan, Hostetler, Burton, et al. (2014) conducted an in-depth studyevaluating all aspects of the CALIPSO aerosol layer products, and foundthat while the CALIPSO layer detection scheme works well duringnighttime measurements, its daytime performance is (as expected)somewhat degraded by the lower signal-to-noise ratios (SNR) thattypify daytime measurements (Hunt et al., 2009). Accurate back-scatter and extinction retrievals of aerosols within these layers inthe CALIPSO algorithm depends upon the selection of the aerosollayers as one of six subtypes which is done using their optical proper-ties and surface characteristics (Omar et al., 2009). The CALIPSO aerosolsubtypes have been compared with corresponding aerosol types de-rived from AERONET data (Mielonen, Arola, Komppula, Kukkonen,et al., 2009; Misra, Tripathi, Kaul, & Welton, 2013) and from highspectral resolution lidar (HSRL) measurements (Burton et al.,2013) with overall good agreement but with some deviations forsmoke and polluted dust.

3. Results

3.1. Examples of pollution outflow in CALIPSO measurements

Fig. 1 shows two examples of CALIPSO browse images of attenuatedtotal backscatter measurements at 532 nm in the vicinity of MCMA onDecember 25, 2008 and December 4, 2012 respectively, with arrowsmarking the position closest to MCMA. The CALIPSO orbit tracks arealso shown, with the closest distance between MCMA and theCALIPSO transect being ~100 km in both examples. The large high-altitude plumes of high backscatter can be clearly seen extending overlarge distances from MCMA (extended yellow-orange features).

On December 25, 2008, the plume extendsmostly northwardwith athinner plume extending to south. On the other hand for December 4,2012, most of the plume seems to be extending to the south of thecity. Because other strong sources of particulate matter are absent atthis time of the year, these are likely to be signatures of anthropogenicpollution outflow from the city itself, possibly with some contributionfrom the Popocatépetl volcano. As can be seen, distinct outflow plumesreach altitudes of up to ~5 km, and are lofted several kilometers abovethe mountainous terrain. Pollution from MCMA area gets vented bythermally induced winds and convection acting like an “air-pump”and then advected away by synoptic winds (de Foy et al., 2006, 2011).In theCALIPSO algorithmscene classification algorithms, optical proper-ties such as volume depolarization ratio and attenuated backscattercolor ratio are used in conjunction with spatial information to distin-guish aerosol layers from clouds (Liu et al., 2009). The vertical featuremasks in the bottom panels of Fig. 1 indicate that in each case theoutflowing plumes are aerosol layers with a few cloud layers embeddedin them. These strong aerosol plumes are clearly detected in theCALIPSO data in the winter months of every year on transects passingin the vicinity of MCMA, which implies that this is a robust, persistentphenomenon and not an artifact of the measurements as such. In prin-ciple, if these plumes represent pollution outflow from the MCMA re-gion, they should be observable during other seasons as well.However, overlying clouds during summer and biomass burning emis-sions during spring transported from the Yucatan province may notallow for unambiguous identification of these plumes.

Figs. 2 and 3 show the back trajectories for air parcels initiated alongthe CALIPSO transects on the two event days that can be traced back towithin 20 km of MCMA within about 24–60 h. These trajectories weredriven by GEOS5 winds (Reinecker, Suarez, Todling, Bacmeister, et al.,2008) and calculated using the NASA Langley Trajectory Model(LaTM) (Fairlie, Szykman, Gilliland, Pierce, et al., 2009; Pierce & Fairlie,1993). The trajectories are color coded according to their initial altitudeswhich are given in the legend in the inset of the figures. For the event onDec. 25, 2008, a significant part of the high altitude aerosol plume can betraced to the vicinity of MCMA. The air parcels spent a long time nearthe city which would allow the pollution to be picked up. For theevent on Dec. 4, 2012, a stronger north-easterly flow prevailed and asmaller part of the high altitude aerosol plume can be directly relatedto locations within 20 km of the city.

3.2. Optical properties of the outflow

Retrieved optical properties of the outflow are shown in Fig. 4a withheight-latitude cross sections of particulate depolarization ratio at532 nm, an indicator of particle shape. We have used all data within acoincidence strip between 98W and 100W for thewintermonths (No-vember through February) between 2007 and 2012 for this plot. Thedatawas limited to nighttime transects only as the signal-to-noise ratiosare significantly worse during the daytime because of the solar back-ground illumination. For this plot, we have calculated theparticulate de-polarization ratio as [βp / (βT − βp)], where βp is the perpendicularbackscatter coefficient andβT is the total backscatter coefficient as avail-able in the CALIPSO profile products. We also calculate the uncertainty

Fig. 1. Examples of aerosol outflow fromMexico City area in CALIPSO lidar data on two different days. The top panels show the 532 nm total attenuated backscatter measured by the lidarand the bottom panels show the vertical feature masks. The corresponding CALIPSO transects are shown in the insets in the top panels.

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in the particulate depolarization ratio by propagating the uncer-tainties using the above equation. To ensure reliable retrievals, thedata were filtered using the extinction quality control (ext_qc) pa-rameter reported in the CALIPSO level 2 data products. Only thosedata for which the initial estimate of lidar ratio generated a stable ex-tinction solution (ext_qc = 0) or for which lidar ratio estimatescould be made directly from the data (ext_qc = 1) or where theaerosol layer is opaque (ext_qc = 16) were used (Young &Vaughan, 2009). This filtering criterion removes about 2% of the

Fig. 2. Back trajectory paths for parcels that can be traced to within 20 km of Mexico City acorresponding to the colored trajectories are given in the legend. (For interpretation of the referThis plot was generated using Google Earth imagery (data LDEO-Columbia, NSF, NOAA, image

profiles within the coincidence box. Data points where the extinc-tion uncertainty calculation diverged (indicated by a value of 99.99for the estimated uncertainty) were also rejected. Further, only thedepolarization data with a threshold on the estimated relative un-certainty (500%) was used. This threshold was used to include datawith lower depolarization ratios with relatively higher uncertainty,which would otherwise be preferentially removed. After applicationof all the filters, approximately 95% of the profiles remain. The lidardata were binned in 0.25° in latitude.

nd were subsequently sampled by CALIPSO on December 25, 2008. The initial altitudesences to color in this figure legend, the reader is referred to theweb version of this article.)Landsat, data SIO, NOAA, U.S. Navy, NGA, and GEBCO).

Fig. 3. Back trajectory paths for parcels that can be traced to within 20 km of Mexico City and were subsequently sampled by CALIPSO on December 4, 2012. The initial altitudes corre-sponding to the colored trajectories are given in the legend. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)This plot was generated using Google Earth imagery (data LDEO-Columbia, NSF, NOAA, image Landsat, data SIO, NOAA, U.S. Navy, NGA, and GEBCO).

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The aerosol particulate depolarization ratio clearly shows enhancedvalues (~0.2) in the plume heading northward and also to some extentto the south albeit with lower values, as compared to the valleys andover the oceanic areas to the south of MCMA (the location of MCMA is

Fig. 4. a) Height latitude distribution of particulate depolarization ratio and b) Variation ofparticulate depolarization ratio along the outflow.

marked by a vertical line in Fig. 4a). Note in particular the differencein depolarization as a function of height near 22°N with the lowdepolarizing aerosols near the ground over valley areas and highdepolarizing aerosols aloft. The outflow fromMCMA as seen in the gen-erally enhanced particulate depolarization ratios can extend up to 25°–28°N, thus reaching the coastal areas of Texas, and once again indicatingthe regional influence of the outflow from MCMA. CALIPSO lidar mea-surements thus confirm and extend the results from the several fieldmissions carried out over the MCMA region (Molina et al., 2007, 2010;Voss, Zaveri, Flocke, Mao, et al., 2010). Fig. 4b shows the mean particu-late depolarization ratio between 2.5–5 km and 98°W–100°Was a func-tion of latitude in the outflow. There is a tendency for increasingdepolarization as the plume evolves away from the MCMA region inthe northward direction. The reason for this is not entirely clear at thistime. In general with aging, the aerosol particles in the plume may beexpected to become more spherical by chemical processing or mixingwith other aerosols injected locally during the transport. The increasingtendency in depolarizationmay imply that the pollution (mostly spher-ical particles) originating from Mexico City becomes thinner as theplume is transported further north and dusty non-spherical particlesgenerated locally along the transport path become more significant, al-though the extinction values (see Fig. 8) suggest that the aerosol loadingis very small at high altitudes in the downwind region. In absence of sig-nificant mixing (Voss et al., 2010), it is also possible that the sphericalparticles in the plume from Mexico City may fall off faster than thedusty non-spherical particles, resulting in increased overall depolariza-tion as the plume transports away fromMCMA. Such a shape dependentsegregation has been observed in the Saharan Air Layer (SAL) fromCALIPSO data (Yang, Marshak, Varnai, Kalashnikova, & Kostinski,2012). In contrast the aerosols transported towards the south seem todecrease in depolarization,whichmay be understood in terms of hydra-tion as the transects approach the ocean quickly in this direction.

Fig. 5 shows the histogram of mean particulate depolarization ratioin the vicinity of MCMA. Here we have used a coincidence box of 19°–20°N and 98.5°W–99.5°W and altitudes of 2.5–5 km. The particle depo-larization ratio distribution shows a rather broad peak near 0.1–0.15.The generally high values of the aerosol depolarization ratio in the out-flow plume indicate the presence of non-spherical particles like dust inthe vicinity of MCMA and are quite consistent with previous measure-ments. Using the NASA Langley airborne High Spectral ResolutionLidar (HSRL) measurements as part of the MILAGRO (Megacity Initia-tive: Local and Global Research Observations) campaign, Rogers, Hair,

Fig. 5. Histogram of particulate depolarization ratio in the vicinity of MCMA. Fig. 7. Histogram of particulate color ratio in the vicinity of MCMA.

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Hostetler, Ferrare, et al. (2009) found evidence of two distinct aerosoltypes in theMCMAoutflow regionwithparticulate depolarization ratiosof ~0.1 and ~0.2 in the two types, which is similar to the CALIPSOmea-surements shown here. In general, Rogers et al. (2009) infer a non-spherical component in most of their measurements. de Foy et al.(2011) also found significant impact of dust in the MCMA region.While the urban aerosols and biomass burning smoke particles havelow depolarization ratios near the sources, the aerosols rapidly mixwithmainly dust as the plume evolves into amixed aerosol type (within~10h). The resulting depolarization ratio of themixed plume is ~0.1 (deFoy et al., 2011), similar to the depolarization ratios observed in the out-flow plume observed by CALIPSO (Fig. 4). Note also that the generallyhigh depolarization ratio implies that the aerosols are probably notdominated by volcanic inputs—in general MCMA is subject to SO2 emis-sions from the Popocatépetl volcano near the city and the sulfatesresulting from these emissions are likely to have low depolarization ra-tios (de Foy et al., 2009).

Figs. 6 shows the corresponding distribution of the particulate colorratio near MCMA. The particulate color ratio (ratio of backscatter at1064 nm/532 nm) is an indicator of particle size and was calculatedon a profile-by-profile basis and later averaged. In addition to the qual-ity assurance filters employed for 532 nmdata as described above, sim-ilar filters (ext_qc = 0, 1 or 16) were used for the 1064 nm data. Thecolor ratios tend to show somewhat lower values in the outflowplume as compared to the oceanic areas (below ~1 km) to the south

Fig. 6. Height latitude cross section of particulate color ratio in MCMA outflow.

of MCMA, although patches of high values can be discerned in thenorth. This is consistent with the fact that marine aerosols are generallycoarser compared to the anthropogenic particles.

Fig. 7 shows the histogram of the particulate color ratio near MCMAfor the same coincidence box as in Fig. 5 (i.e., 19°–20°N and 98.5°W–99.5°W and altitudes 2.5–5 km). The particulate color ratio has asharp peak near 0.7–0.8 suggesting that smaller particles dominatethe size distribution during thewinter months. This color ratio distribu-tion is generally consistent with the backscatter color ratios for mixedplumes (biomass burning smokewith dust) measured by the HSRL dur-ing theMILAGROmission inMarch 2006 (de Foy et al., 2011). Note thatde Foy et al.(2011) define the backscatter color ratio (ratio of backscat-ter at 532 nm to the backscatter at 1064 nm) as the inverse of the colorratio defined here, so the relatively high backscatter color ratio mea-sured by HSRL for the mixed plumes correspond to low color ratios inCALIPSO retrievals.

Fig. 8 shows the height latitude distribution of the 532 nmextinctioncoefficients in the outflow. The extinction coefficient is an extensiveproperty that depends on both aerosol type and aerosol concentration.As can be seen in Fig. 8, the optical thickness of the plume is highestclose to the location ofMCMA, once again pointing to the city as the like-ly origin of these plumes. However, the outflow dilutes sharply awayfrom the city and can be clearly discriminated from the high valuesfrom local sources in the low altitudes over the valley, north or southof the city.

Fig. 8. Height latitude cross section of 532 nm extinction coefficient (km−1) near MCMA.

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3.3. Aerosol subtypes and outflow estimate

CALIPSO extinction retrievals need information on the lidar ratio(i.e., extinction-to-backscatter ratio) associated with the aerosol sub-types inferred for each layer on the basis of optical properties as wellas the characteristics of the underlying surface (Omar et al., 2009).These aerosol subtypes, although not exhaustive, provide valuable in-formation on the aerosol environment. Fig. 9 shows overall distributionof the dominant aerosol subtypes between 2007 and 2012 for the win-ter months (November–February) in the vicinity of MCMA.

For this figure representing the subtypes in the vicinity ofMCMA,wehave used a coincidence box of 19°N–20°N and 98.5°W–99.5°W andhave included only those layers detected at 5 km horizontal resolutionoccurring with layer base height above 2.5 km and layer top heightbelow 5 kmwhichwere classified as aerosols with feature classificationquality assurance flag of medium or high and cloud-aerosol discrimina-tion (CAD) score between−20 and −100. Using only those layers de-tected at the 5 km horizontal resolution restricts the data set to onlythe highest SNRmeasurements and thus provides increased confidencein the aerosol subtyping results. As seen in this Figure, in all months,polluted dust, which is the model for a mixture of smoke and dust,was the dominant aerosol subtype with a maximum of ~80% in Febru-ary. The distribution of the particulate depolarization ratio for these pol-luted dust layers does not show any significant dependence on theoverlying attenuated backscatter. Such a dependence might potentiallyimpact the classification of polluted dust layers (Burton et al., 2013).Smoke layers also contributed substantially, with a highest fraction of~35% in November. Dust accounted for about 10%with 4–12% contribu-tion coming from clean layers. Previous results from HSRL measure-ments during the MILAGRO campaign (March 2006), indicated thepresence of primarily three aerosol types, i.e., dusty mix, smoke andurban pollution, the dusty mix being similar to the polluted dust (mix-ture of dust and smoke) in the CALIPSO algorithm (Patadia, Kahn,Limbacher, Burton, et al., 2013).

CALIPSO lidar measurements, as presented here clearly show largeplumes of aerosol emanating from MCMA region during the wintermonths, which are likely to have significant regional scale impacts. Itis of some interest, therefore to estimate the amount of mass outflowin these plumes. An approximate estimate of the mass flux of pollutionoutflow can be derived using the following equation (Kaufman et al.,2005):

M ¼ τ=αð Þ L v g s−1� � ð1Þ

where, τ is the optical depth estimated between the outflow altitudes,α(m2 g−1) is the dry mass extinction efficiency of the aerosols, L (m) is a

Fig. 9. Relative abundance of the various aerosol subtypes in the vicinity of MCMA.

characteristic length scale of the MCMA in the east–west direction acrosswhich the outflow occurs and v (m s−1) is the northwardwind speed. Toobtain a first order estimate of the outflow for the two events shown inFig. 1, we have used the mean extinction profile in the vicinity of MCMA(using all cloud free profiles between 19°N–20°N and 98.5°W–100.5°Wusing the quality filters mentioned earlier) and computed the opticaldepth between 2.5 and 5.0 km. Within this coincidence box, theCALIPSO level 2 aerosol products report 11 cloud-free 5-km profiles forDecember 25, 2008 and 17 for December 4, 2012. The median dry massextinction efficiency values derived from an ensemble of 20 aerosolmodels is 0.95 m2 g−1 for dust, 5.7 m2 g−1 for organic material and9.0 m2 g−1 for black carbon (Kinne, Schulz, Textor, Guibert, et al., 2006).Yu, Remer, Chin, Bian, et al. (2008) have used a value of 4 m2 g−1 forAsian outflows with low dust component. As discussed above, the domi-nant aerosol subtype identified by CALIPSO in MCMA region is polluteddustwhile severalfield campaigns indicate the strong influence of organicaerosols with the organic aerosol loading increasing as the plume istransported away from MCMA (Molina et al., 2007). We assume a valueof 5m2 g−1 forα to represent amixture of dust and smoke (or black car-bon). Further, adopting L = 40 km for the extent of MCMA in east–westdirection and using the daily averaged gridded northward wind speedas available from the National Center for Environmental Prediction(NCEP) near MCMA at 700 hPa (to represent the outflow speed between2.5 and 5 km), we obtain approximate northward outflow amounts of~(+)180 tons per day and ~(−)610 tons per day for the events on De-cember 25, 2008 andDecember 4, 2012 respectively, assuming a constantoutflow over 24 h. The dominant wind direction at 700 hPa on December4, 2012 was southward and the mass flux is calculated with a negativesign, which is also consistent with the browse image, with the bulk ofthe outflow occurring to the south. We have not taken into account theeffect of aerosol swelling with increasing humidity, as in winter theMCMA area is generally dry. The relative uncertainties in the various pa-rameters in Eq. (1) are not well-known. Assuming conservative valuesof 50% for the uncertainty in the mass extinction efficiency (α), 10% forthe uncertainty in L and 20% for the uncertainty in NCEP daily windsused here, we estimate an uncertainty of ~50% for the outflow amounton bothdays bypropagating the uncertainties in the extinction coefficientprofiles within the coincidence box. If the episodic outflows are dominat-ed by black carbon, the mass outflow amount will decrease significantly(by a factor of 5/9 if we useα=9m2 g−1 for black carbon). The accuracyof these mass outflow estimates is difficult to gauge from the currentlyavailable information near the MCMA. Kunkel, Lawrence, Tost, Kerkweg,et al. (2012) have modeled impact of aerosol transport from 46 mega-cities including the MCMA. However, they used a constant emission ratefrom all cities and modeled the annual total mass deposition flux usingthe prevailing meteorology, so it is not possible to directly compare ourmass outflowvalueswith theirmass deposition values. Despite theuncer-tainties the values reported here might represent a first order estimate ofthe particulate outflow episodes from a major megacity using satellitedata on aerosol extinction, and are presented essentially as a possible ap-plication of CALIPSO lidar data for studies of regional impacts of megacitypollution.

4. Conclusions

We have used CALIPSO lidar measurements to detect regional scalepollution outflows from MCMA during winter. Strong outflows wereclearly detected emanating from the MCMA and often flowed as farnorth as the Texas coast. These measurements of long-range transportofMCMApollution are in agreementwith theMILAGROballoon and air-craft measurements in the Gulf of Mexico. The optical properties of theaerosols as probed by the lidar indicate presence of non-spherical parti-cles with depolarization ratios near 0.1. The CALIPSO algorithm identi-fied most of these aerosols as polluted dust, which is consistent withprevious lidar measurements in the vicinity of MCMA. A first order esti-mate of the mass outflow on two occasions indicates that outflow

211J. Kar et al. / Remote Sensing of Environment 169 (2015) 205–211

amounts can be of the order of several hundred tons per day. This satel-lite perspective confirms the strong regional scale impact of the pollu-tion outflows from MCMA. This study indicates the potential of usingCALIPSO lidar data for monitoring the regional air quality around largeurban centers like MCMA and the impact of long range pollutionoutflows.

Acknowledgments

The CALIPSO data used in this work were obtained from the NASALangley Atmospheric Sciences Data Center. J.K. acknowledges useful dis-cussions with B. de Foy. We thank Kurt Severance for the help with theCALIPSO curtain plots.

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